1. Field of the Invention
The present invention relates to a novel process for the preparation of dual-cure hardeners, as well as to coating systems which contain these hardeners, and the use of these coating systems.
2. Description of the Prior Art
The dual-cure hardeners obtainable according to the invention arise from the reaction of certain hydroxyalkyl (meth)acrylates with polyisocyanates. (Meth)acrylates within the meaning of the present invention are esters of acrylic acid or methacrylic acid and mixtures thereof. Hydroxyalkyl (meth)acrylates are used, inter alia, for the preparation of hardeners for two-component or multi-component coating systems which cure by radical polymerisation and addition reaction of isocyanates with suitable co-reactants. The radical polymerisation can here be initiated by actinic radiation. The combination of hardening by two reaction mechanisms is designated “dual-cure” by those skilled in the art.
Advantageous dual-cure systems contain molecules which have available both one or more isocyanate groups and at least one radically polymerisable double bond. Such molecules are designated dual-cure hardeners by those skilled in the art. Such hardeners are generally prepared by the reaction of polyisocyanates with hydroxyalkyl (meth)acrylates. In order to obtain particularly highly cross-linked coatings, it is expedient to use hardeners which have high functionality in particular in terms of radically polymerisable double bonds, and which can be advantageously prepared by the use of hydroxyalkyl (meth)acrylates which have a high functionality in terms of acrylate and/or methacrylate groups. In order to obtain a targeted molecular structure of the dual-cure hardener, it is furthermore expedient to use hydroxyalkyl (meth)acrylates whereof the hydroxy-functionality is distributed as narrowly as possible and is close to one. It is possible as a result of this targeted molecular structure to provide, inter alia, hardeners which are distinguished by a low viscosity. Therefore the preparation of the hydroxyalkyl (meth)acrylates by esterification of polyols such as trimethylolpropane or pentaerythritol with (meth)acrylic acid as a process which proceeds randomly and generates a broader distribution of the hydroxy-functionality is disadvantageous. Furthermore, by-products having higher molecular weight, which have been identified as products of an addition reaction between hydroxyl groups and the C-C double bonds of acrylates, for example in Analytical Sciences, November 2001, Vol. 17, pp. 1295-1299, frequently form.
DE-A 19 860 041 discloses 3-acryloyloxy-2-hydroxypropyl methacrylate as a possible compound for a reaction with polyisocyanates for the preparation of dual-cure hardeners. These are prepared by catalysed allophanation with an excess of hexamethylene diisocyanate (HDI) followed by distillation of the excess HDI. However, because of the risk of spontaneous polymerisation, the distillation of HDI in the presence of acrylate groups or methacrylate groups is highly critical, and industrially—if this is indeed possible at all—is associated with very high costs and is therefore uneconomic. This specification makes no mention of the preparation or purification, or of the necessary purity or sourcing, of 3-acryloyloxy-2-hydroxypropyl methacrylate.
The literature generally describes the preparation of 3-acryloyloxy-2-hydroxypropyl methacrylate by the reaction of glycidyl methacrylate with acrylic acid, both commercially available at high purity, with suitable catalysis. No data regarding the purification/purity of the product are provided. For instance, EP-A 0 900 778 describes the reaction of excess acrylic acid following esterification reactions with glycidyl methacrylate catalysed with benzyltriethylammonium chloride. Although 3-acryloyloxy-2-hydroxypropyl methacrylate is commercially obtainable from fine chemicals suppliers (Sigma-Aldrich GmbH, Steinheim, Germany), the purity determined by gel permeation chromatography is below 50 wt. %, in particular undesirable constituents of higher molecular weight are found in the product. The preparation process is unknown. When the commercially obtainable product, and also the product prepared by benzyltriethylammonium chloride catalysis, is reacted with polyisocyanates to obtain dual-cure hardeners, contrary to expectations stable products are not obtained (see Comparison Examples V11 and V3).
After purification by distillation 3-methacryloyloxy-2-hydroxypropyl methacrylate of greater than 85 wt. % purity is marketed by Röhm, Darmstadt, Germany. The stability of the product is low, the product must be stored in a cool place, thus hampering use on an industrial scale. The stability of reaction products of this 3-methacryloyloxy-2-hydroxypropyl methacrylate with polyisocyanates to obtain dual-cure hardeners is very low (see Comparison Example V12).
The object was therefore to provide a process by which storage-stable reaction products of, for example, 3-acryloyloxy-2-hydroxypropyl methacrylate with polyisocyanates can be obtained which are suitable as hardeners in dual-cure coating systems.